Apparatus and methods for repairing tenons on turbine buckets

ABSTRACT

Bucket tip tenons requiring refurbishment are removed from the bucket tips by grinding. Groups of buckets are clamped on a rotatable table in positions adjacent an induction heating coil and a robotic welding arm, respectively. While one group is preheated, the other group of buckets is in a welding location, enabling the welding head on the robotic arm to sequentially apply weld buildup material to the bucket tips. After applying weld buildup material in multiple passes and sequentially to the bucket tips, the buckets are removed, stress-relieved and the weld buildup material machined to provide refurbished tenons. Upon removal of the buckets from the table, the table is rotated to locate the preheated buckets in welding locations and additional buckets are clamped to the table for preheating.

BACKGROUND OF THE INVENTION

The present invention relates to apparatus and methods for repairingdamaged tenons on the tips of turbine buckets and particularly relatesto apparatus and methods for repairing eroded tenons with reducedmanufacturing cycle time, costs, labor and improved quality.

Turbine buckets, for example, steam turbine buckets, are typicallyprovided with an annular cover overlying the tips of the buckets. Thecovers are used to dampen and stiffen the response of the buckets andalso provide a circular band on which a seal can be provided. The coversare conventionally provided in arcuate segments overlying a plurality ofbucket tips. In the originally manufactured bucket, one or more tenonsare provided projecting from the tip of the bucket and are received incorresponding openings in the cover. The tenons are then peened orotherwise secured to the covers to retain the covers on the buckets.Over time and extended usage of the turbine, particularly steamturbines, the bucket tenons corrode away, requiring the bucket/coverassembly to be refurbished by repairing the tenons and resecuring thecover to the bucket tips.

Previously, to repair bucket tenons, the buckets were removed from theturbine with each individual bucket being placed on a workbench. Thecorroded tenon was then ground away down to a nub on the tip or flushwith the bucket tip. The tenon tip was preheated and cleaned. Weldmaterial was then applied to the tip manually in multiple passes, payingstrict attention to the magnitude of the preheat. The weld buildup wasthen stress-relieved and machined to form the new or repaired tenons.This prior process was manually performed, tedious and costly, resultingin very low production rates.

BRIEF DESCRIPTION OF THE INVENTION

In accordance with a preferred embodiment of the present invention,there is provided apparatus and methods for reducing manufacturing cycletime, costs and labor and improving quality by automating the repairprocess. In an aspect hereof, a fixture, preferably a table, includes aplurality of clamps for clamping groups of buckets to the table, forexample, in two groups of four buckets each. The buckets are thusclamped in predetermined positions on the table. One group of buckets ispreheated, preferably by an induction coil, while weld material isapplied sequentially to the tips of the buckets of the other group inmultiple passes using a robotically carried welding head. Weld materialis applied in multiple passes after temperature measurements are madefor each bucket tip such that each bucket tip has sufficient preheat toenable a further welding pass. Upon completing the weld buildup for thegroup of buckets, the buckets are removed from the table fixture, placedin a further fixture in an inverted position and disposed in a fluidizedbed for stress relief. The table fixture is also displaced to locateanother group of preheated buckets into welding positions relative tothe welding head carried by the robotic arm such that the weld materialmay be applied sequentially to the next group of preheated bucket tipsin multiple passes. Substantially simultaneously, a further group ofbuckets are clamped to the table fixture and preheated by the inductioncoil. The process is repeated until all of the damaged tenons have beenprovided with sufficient weld build-up and stress-relieved. Afterstress-relief, the weld buildup at each bucket tip is machined,preferably by CNC machines, to form replacement tenons.

It will be appreciated that the buckets may be initially fixed inpredetermined positions by individual clamps and not necessarily on arotatable table. Thus, after preheating, the weld material may beapplied robotically and sequentially to the preheated bucket tips inmultiple passes and subsequently stress relieved and machined. Thestress relief may be accomplished by any one of a variety of knowntechniques including the technique disclosed specifically herein.

In a preferred embodiment according to the present invention, there isprovided a method of repairing tenons on the tips of turbine buckets,comprising the steps of (a) releasably securing a plurality of bucketsto a fixture in predetermined positions, (b) preheating the turbinebuckets, (c) applying weld material sequentially to the tips of thepreheated buckets secured to the fixture in a first weld pass, (d)applying additional weld material sequentially to the tips of thebuckets secured to the fixture in at least one additional pass, (e)stress-relieving the buckets and (f) machining the applied weld materialto form tenons on the bucket tips.

In a further preferred embodiment a method of repairing tenons on thetips of turbine buckets, comprising the steps of: (a) fixing a pluralityof buckets in predetermined positions; (b) preheating the turbinebuckets; (c) applying weld material sequentially to the tips of thepreheated buckets secured to the fixture in a first weld pass while thebuckets are fixed in the predetermined positions; (d) applyingadditional weld material sequentially to the tips of the buckets in atleast one additional pass while the buckets are fixed in predeterminedpositions; (e) stress-relieving the buckets; and (f) machining theapplied weld material to form tenons on the bucket tips.

In a further preferred embodiment according to the present invention,there is provided an assembly for applying weld material to tips ofturbine buckets to form one or more tenons on said bucket tips,comprising a fixture having clamps for releasably securing first andsecond groups of buckets in predetermined positions on the fixture, aninduction coil for preheating the groups of buckets, a welding head forapplying weld material to the tips of the buckets, the fixture beingmovable between first and second positions, the fixture in the firstposition thereof locating the first group of buckets adjacent the weldhead enabling weld material to be applied to the tips of the bucketsthereof and the second group of buckets adjacent the induction coil forpreheating the bucket tips thereof, the fixture in the second positionlocating the second group of buckets adjacent the welding head, enablingweld material to be applied to the tips of the buckets of the secondgroup thereof.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a fragmentary schematic illustration of an apparatus forproviding weld build-up on the tips of turbine buckets;

FIG. 2 is a fragmentary perspective illustration of the step of clampinga bucket to the fixture illustrated in FIG. 1;

FIG. 3 is a view similar to FIG. 2 illustrating a bucket tip clamped tothe fixture;

FIG. 4 is a view similar to FIG. 1 illustrating the application of theinduction heating coil and the weld build-up provided by the weldinghead carried by the robotic arm of an industrial robot;

FIG. 5 is an enlarged illustration showing the weld build-up on the tipof a bucket tip;

FIG. 6 is a perspective view of a further fixture for holding thebuckets with the weld build-up on the tips in an inverted position;

FIG. 7 is a view similar to FIG. 6 illustrating the immersion of theweld build-up on the bucket tips into a fluidized bed for stress relief;

FIG. 8 is an enlarged schematic fragmentary view illustrating a weldbuild-up on the tip of a bucket;

FIG. 9 is a schematic illustration of a CNC machine grinding the weldbuild-up to form tenons; and

FIG. 10 is a perspective view of the bucket tip illustrating thefinished tenons on the tip.

DETAILED DESCRIPTION OF THE INVENTION

Referring to the drawings, particularly to FIG. 1, there is illustratedan assembly, generally designated 10, for preheating and applying weldbuild-up material on the tips of turbine buckets at a production ratewith increased quality and reduced repair cycle time. The assemblyillustrated in FIG. 1 includes a fixture 12 comprising a table 14mounted for rotation about a vertical axis. The assembly also includesan induction heater coil 16 and a robot 18 mounting a welding head 20.

Particularly, table 14 includes a plurality of upstanding mechanicalover-center clamps 22. The clamps are mounted on uprights 24 carried bytable 14 and include a stop 26 and movable arms 28 (FIGS. 2 and 3). Theclamps 22 are per se conventional and are used to clamp the upper endsof the buckets 30 against the stop 26 in predetermined positions. Itwill be appreciated that the bucket 30 includes a platform 32 and adovetail 34, the latter dovetail 34 resting on the top of the table 14,while the upper end of the bucket 30 is clamped between the arms 28 andstop 26 as illustrated in FIG. 3. The clamps 22 are provided withhandles 36 by which the clamps can be manually positioned to clamp thebuckets in the predetermined positions as illustrated in FIG. 4.

As illustrated in FIG. 1, the clamps 22 are arranged in two groups offour clamps each along opposite sides of the table 14. It will beappreciated that a fewer or greater number of clamps 22 in each group(and hence buckets 30) may be provided and that the groups of clampsneed not be aligned along opposite sides of the table but may be alignedat other angular orientations relative to one another. The table 14 ispivotable about a central vertical axis to locate the groups of clampsand, hence, the clamped buckets in respective positions adjacent theinduction heater coil 16 and the robot 18 and, particularly, the weldinghead 20 carried by robot 18. In the illustrated preferred embodiment,the clamps are mounted along opposite sides of the table 14 and,accordingly, the table 14 is rotatable 180° between positions locatingeach group of buckets adjacent the induction heater coil 16 and adjacentthe welding head 20 in welding positions such that the welding head 20can provide weld buildup on the tip of each bucket in each group. Thetable 14 may be locked in each of the two rotatable positions by cornerpins 40 locking the table to fixed stanchions.

The induction heater coil 16 lies to one side of the table 14 adjacent agroup of buckets clamped to the table by clamps 22. The induction heatercoil 16 preheats the buckets without contact with the buckets wherebythe cross-sections of the airfoils adjacent the bucket tips aresubstantially uniformly heated. Preferably, the induction coil preheatsthe bucket airfoils to a temperature of about 450° F. The robot 18 maybe of conventional construction, including a manipulator arm 42, acontroller 44 for controlling the movement of the welding head 20carried at the tip of the arm and a gas cup 46 for providing the inertgas atmosphere for providing weld buildup on the tips of the turbinebuckets. It will be appreciated that the robot is programmable to applyweld material 43 onto the tips of the buckets when the bucket tips arelocated in welding positions. Thus, the robot can be programmed, i.e.,taught to apply welding material sequentially to the bucket tips at thevarious locations of those bucket tips clamped to table 14 along the oneside thereof. Additionally, and for reasons noted below, the robot alsocarries an infrared scanner 50 such that the temperature of each buckettip can be ascertained prior to welding. The robot 18 may be of the typemanufactured and sold by Daihen, Inc. of Tipp City, Ohio and identifiedunder their trade name of OTC-Almega X Series.

Referring to FIGS. 6 and 7, there is also provided an additional fixtureor support 60 for supporting the buckets in inverted positions with theweld buildup material on the depending tips of the bucket. Thus, whensufficient weld buildup material has been applied to a group of buckets,the buckets are manually removed from the clamped positions on the tableand inserted into the support 60. In FIG. 7, the support 60 isillustrated with the bucket tips, including the weld buildup, immersedin a fluidized bed 62. The fluidized bed 62 may be of the type includinga container holding small metal or ceramic beads which are fluidizedwithin container 62 by heated air supplied to the bottom of thecontainer 62 for flow upwardly in and about the beads and outwardlythrough the top of the container. The supply of preheated air,schematically illustrated at 64, may be provided by an electrical or gasfurnace, not shown, and blown into the container 62. By dipping tips ofthe buckets including the weld buildup material on each bucket tip intothe container 62, the high temperature stress-relieves the buckets bythe application of a substantially uniform temperature to the buckets.Once the stress-relief has been performed, the weld buildup illustratedin FIG. 8 can be subsequently machined, e.g., by a CNC machine,schematically illustrated at 70 in FIG. 9 to form the bucket tenons 76illustrated in FIG. 10.

A description of the process of refurbishing the tenons on the bucketswill now be described. As will be appreciated, over extended time andusage of the turbine, particularly steam turbines, the originallymanufactured tenons abrade and erode away, requiring the bucket to berefurbished to provide new tenons so that the covers may be secured tothe refurbished buckets. Thus, the buckets with the eroded or corrodedtenons are removed from the turbine and the damaged tenons removed fromthe bucket tips by a grinding operation, not shown. Once the bucket tipshave been ground to a substantially flat configuration, the buckets areready for refurbishment.

The buckets are disposed in an upright orientation on table 14 withtheir dovetails 34 resting on the table. Upper end portions of thebucket airfoils are clamped by clamps 22 into predetermined positions onthe table. In the illustrated embodiment, a first group of buckets areclamped along one side of the table, for example, the side of the tableadjacent the induction heater coil 16. The buckets are clamped inpredetermined positions corresponding to positions preprogrammed intorobot 18 such that the robot will apply weld buildup at predeterminedwelding locations upon rotational movement of the first group ofpreheated buckets into a welding location adjacent the welding head 20.With the first group of buckets clamped in predetermined positionsadjacent the induction heating coil 16, coil 16 is actuated to preheatthe buckets to a predetermined temperature, for example, 450° F. Uponreaching the predetermined temperature, the table 14 is rotated aboutthe vertical axis, in this instance about 180°, to locate the preheatedbuckets in the welding location. Once the table has been rotated, asecond group of buckets are clamped to the table along the side thereofadjacent the induction heating coil 16. It will be appreciated thatwhile the first group of buckets are in the welding location and weldbuildup material is being applied to the bucket tips, the second groupof buckets along the opposite side of the table are being preheated bythe induction heating coil 16.

With the first group of buckets in the welding location, the robot 18 isactuated to sequentially apply weld buildup to each of the preheatedbucket tips. Specifically, the robot 18 is actuated to locate thewelding head 20 including the gas cup 46, in proximity to the preheatedbucket tip as illustrated in FIG. 5. The infrared scanner 50 scans thebucket tip to ensure that adequate preheat remains in the bucket tip toenable application of weld material. The welding technique may be one ofmany welding techniques. For example, using a MIG system, wire iscontinuously fed through a thimble on the gas cup and CO₂ or argon gasor a mixture thereof is dispersed to cover the weld. This shielding gasdrives away the atmosphere and shields the welding head, enablingapplication of weld material to the bucket tip. Upon applying apredetermined quantity of weld buildup material, the pattern memorycontrol of the robot causes the robot arm to move the welding head fromthe first bucket tip to the second bucket tip of this first group ofbuckets where the process of applying weld buildup material is repeated.The scanner 50 reads the temperature and, if appropriate, weld buildupmaterial is applied to the second bucket tip. The robot is sequenced toapply weld buildup material in a first pass to each of the bucket tipsof the first group of buckets in the predetermined positions on thetable and in the welding locations. The application of weld buildupmaterial is limited to restrict the magnitude of heat being absorbed bythe bucket. The infrared scanner monitors the temperature at each buckettip prior to applying weld material. If the monitored temperature is toohigh, the robot stops the welding sequence until the appropriatetemperature is reached. Conversely, if the tip temperature is too lowfor adequate welding, additional heating may be supplied.

Upon completion of a first pass applying weld buildup material to eachof the bucket tips in the first group of buckets in the weldinglocations, the robot displaces the arm to provide a second pass andadditional weld material along each of the bucket tips. Thus, additionalweld buildup material is provided in sequence to each bucket tip,similarly as the first pass. Thus, multiple welding passes are performeduntil the required buildup of weld material has been achieved.

It will be appreciated that the sequencing and fixturing provides twofunctions. First, it positions the welding head at the start of eachweld at the same location for each bucket tip, thus ensuring consistencyof the applied weld material. Further, it enables adequate time for theheat generated during the welding process to dissipate and travelthrough the material by conduction. This timing and sequencing, alongwith the monitoring of the bucket temperature, provides the temperaturestability control, enabling an increase in production rate.

Once the weld buildup is complete on the first group of buckets, thebuckets are removed from the table 14, inverted and located in thesupport 60. Upon removal of the first group of buckets from the table14, the table is rotated to locate the next group of buckets on theopposite side of the table in the welding location. That second or nextgroup of buckets has been preheated to proper welding temperature andthe welding process is repeated. Additional buckets are also located inthe now empty predetermined positions adjacent the induction coil 16 topreheat those additional buckets as the previously preheated group ofbuckets is being welded.

The first group of buckets removed from the table 14 with appropriateweld buildup are located in support 60. Support 60 supports the bucketsin inverted positions, enabling the buckets to be dipped into thefluidized bed 62 for stress relief. After stress relief, for example,for about twenty minutes, the buckets are removed from the fluidizedbed, wrapped in an insulating blanket, not shown, and slowly cooled.After cooling, the weld buildup material on the bucket tips issubsequently machined, for example, by a CNC machine, to form tenons 76.After machining, the bucket is now fully refurbished and restored,enabling bucket covers to be applied to the bucket tips.

While the invention has been described in connection with what ispresently considered to be the most practical and preferred embodiment,it is to be understood that the invention is not to be limited to thedisclosed embodiment, but on the contrary, is intended to cover variousmodifications and equivalent arrangements included within the spirit andscope of the appended claims.

1. A method of repairing tenons on the tips of turbine buckets,comprising the steps of: (a) releasably securing a plurality of bucketsto a fixture in predetermined positions; (b) preheating the turbinebuckets; (c) applying weld material sequentially to the tips of thepreheated buckets secured to the fixture in a first weld pass; (d)applying additional weld material sequentially to the tips of thebuckets secured to the fixture in at least one additional pass; (e)stress-relieving the buckets; and (f) machining the applied weldmaterial to form tenons on the bucket tips.
 2. A method according toclaim 1 wherein step (b) is performed by applying an induction heatercoil to the buckets while the buckets are secured to the fixture.
 3. Amethod according to claim 1 wherein the fixture includes at least firstand second groups of bucket positioning devices for securing first andsecond groups of said buckets, respectively, to the fixture in saidpredetermined positions, locating the first group of buckets secured tothe fixture in said predetermined positions in a welding locationrelative to a welding head to enable application of weld materialsequentially to the tips of the first group of buckets and displacingthe fixture to locate the second group of buckets secured to the fixturein said predetermined positions in said welding location relative to thewelding head to enable application of weld material sequentially to thetips of the second group of buckets.
 4. A method according to claim 3wherein the step of displacing includes rotating the fixture to alignthe bucket tips of the second group of buckets in said predeterminedpositions in said welding location for welding by the welding head.
 5. Amethod according to claim 3 including robotically advancing the weldinghead to apply weld material sequentially to the bucket tips of therespective first and second groups of buckets upon their location insaid welding location.
 6. A method according to claim 1 including, priorto step (e), removing the buckets from the fixture and wherein step (e)includes disposing the bucket tips with the applied weld material into afluidized bed to stress-relieve the buckets.
 7. A method according toclaim 6 including inverting the buckets to orientations with the buckettips facing downwardly, and wherein step (e) includes disposing theinverted buckets in a second fixture and lowering the inverted buckettips carried by the second fixture into the fluidized bed.
 8. A methodaccording to claim 1 wherein step (c) includes robotically controlling awelding head to apply the weld material sequentially to the tips of thebucket in each of said weld passes.
 9. A method according to claim 1including monitoring the temperature of each bucket tip prior toapplying the weld material thereto and controlling the application ofthe weld material to each tip to apply weld material only when themonitored temperature of the tip lies within a predetermined range oftemperatures.
 10. A method according to claim 1 wherein the fixtureincludes at least first and second groups of bucket positioning devicesfor securing first and second groups of said buckets, respectively, tothe fixture in said predetermined positions, preheating said first groupof buckets, and simultaneously preheating said second group of bucketswhile applying the weld material sequentially to the preheated buckettips of said first group of buckets.
 11. A method according to claim 1including providing a robot having an arm carrying a welding head,robotically locating the welding head in a plurality of predeterminedlocations to apply the weld material sequentially to the bucket tips insaid predetermined positions of said buckets during said first weldpass, and performing step (d) by robotically locating the welding headin said predetermined locations to apply the weld material sequentiallyto the bucket tips in said predetermined positions during saidadditional weld pass.
 12. A method according to claim 1 wherein step (a)includes clamping each bucket against a stop carried by the fixture tolocate the bucket tips thereof relative to said fixture.
 13. A method ofrepairing tenons on the tips of turbine buckets, comprising the stepsof: (a) fixing a plurality of buckets in predetermined positions; (b)preheating the turbine buckets; (c) applying weld material sequentiallyto the tips of the preheated buckets secured to the fixture in a firstweld pass while said buckets are fixed in said predetermined positions;(d) applying additional weld material sequentially to the tips of thebuckets in at least one additional pass while said buckets are fixed insaid predetermined positions; (e) stress-relieving the buckets; and (f)machining the applied weld material to form tenons on the bucket tips.14. A method according to claim 13 wherein step (b) is performed byapplying an induction heater coil to the buckets while the buckets aresecured to the fixture.
 15. A method according to claim 13 includingrobotically advancing a welding head to apply weld material sequentiallyto the bucket tips of the buckets in each of the first and oneadditional weld passes.
 16. A method according to claim 13 including,prior to step (e), removing the buckets from their fixed positions andwherein step (e) includes disposing the bucket tips with the appliedweld material into a fluidized bed to stress-relieve the buckets.
 17. Amethod according to claim 13 including monitoring the temperature ofeach bucket tip prior to applying the weld material thereto andcontrolling the application of the weld material to each tip to applyweld material only when the monitored temperature of the tip lies withina predetermined range of temperatures.
 18. A method according to claim13 including providing a robot having an arm carrying a welding head,robotically locating the welding head in a plurality of predeterminedlocations to apply the weld material sequentially to the bucket tips insaid predetermined positions of said buckets during said first weldpass, and performing step (d) by robotically locating the welding headin said predetermined locations to apply the weld material sequentiallyto the bucket tips in said predetermined positions during saidadditional weld pass.